This isn't the same ``indelible'' smear easily removed in years past with a bit of lemon juice, or a good soap-scrubbing. Mexican scientists and electoral officials claim they have a fraud-beater product that could set a new world standard.

``I don't know if the United States Food and Drug Administration would approve it. But it really is impressive,'' says Dong Nguyen, head of a United Nations technical team here providing advice to the Mexican government.

For years, fraud claims have dogged the Institutional Revolutionary Party's 65-year unbroken grip on power. Domestic and international pressure has pushed the Mexican government into a series of reforms, including spending $730 million on mechanisms designed to make this the most credible election in history. Much of the expense has gone into setting up a new computerized electoral roll. Ninety percent of eligible voters (45.7 million Mexicans) now have photo-identification cards, which include a signature and thumbprint.

But the $200,000 spent on developing and producing a truly indelible ink may be the best confidence-building investment the government has made. ``It could be the most cost-efficient way to avoid the double vote,'' Mr. Nguyen says. Researchers say the ink is safe on skin and that the odor disappears with washing. This reporter unsuccessfully attempted to remove the stain with gasoline, soap, and bleach. After four days, it began to fade.

Coming up with an indelible ink was a challenge for the Federal Electoral Institute (IFE), the government agency running the elections. In November, it solicited samples of indelible ink from companies here. Five companies sent in 54 samples. The IFE then contracted the National School of Biological Sciences at the National Polytechnical Institute here to run tests on the inks.

Scientists found that all of the 54 inks indelible on paper, cloth, or metal could be removed from skin using household solvents. Intrigued by the challenge, they offered to develop an indelible liquid. The IFE gave them three criteria: It must last 10 hours on the voter's thumb, be non-toxic, and cannot damage the skin.

A month later, the secret formula was tested and unanimously approved for production by the IFE General Counsel, comprised of all political parties.

``The indelible liquid is the last of three mechanisms to prevent voters from casting more than one vote,'' says Felipe Solis Acero, the IFE director of electoral organization.

WHEN the voter arrives to cast his ballot, his name must be on the voter roll. Next, his identity is checked against the photo-ID card. If the plastic seal on the card is broken, the card self-destructs. After voting, the card is stamped with a ``V,'' which leaves an indentation in the card without breaking the seal. Finally, the ink is applied to the voter's thumb.

Despite the improvements to the voting process, it may be the ink stain that ultimately provides the most important stamp of credibility. Opposition parties continue to question the integrity of the voter roll, claiming as many as 10 percent of the voters are ``phantom'' names.

And a controversy has erupted over the ease with which fake voter credentials can be obtained. A university researcher published a letter on June 18 showing how easy it was for a colleague to receive two credentials under a false name. The interior minister was not impressed and responded by saying both would be prosecuted for electoral fraud.

With its prestige on the line, the National Polytechnical Institute (which developed the ink) will also be overseeing the ink production and distribution. It is verifying ingredients, monitoring the production process, and will test random samples.

To ensure that the 210,000 bottles of ink are not tampered with during distribution, the lids will be sealed with plastic.

But Mr. Solis observes with a rueful smile, there will still be complaints about the ink. ``After the election, we'll be criticized because nobody can get it off.''